MYW tickets are front loaded towards establishing their dollar value. That means that credit is determined the same way that the original sales price is determined ... but in reverse. When you buy the ticket, you pay less for the additional days you get. When you want to apply the dollar value towards a new ticket, the value is done in reverse. The cheapest addon day is the value of your first remaining unused day, the second cheapest is the value of your second remaining unused day, etc. Disney rightly considers you to have already used your most expensive days 1, 2, etc. so your credit is for the lesser amounts.
If you originally had (as an example) a 10 day base ticket and you now have 2 days remaining, your dollar credit will only equal the cost of adding days 9 & 10 to that original ticket. That could be as low as $6 for a base ticket, slightly more for PH or WPD&M tickets.
Take the old ticket to Guest Relations and let them scan it. They will tell you what your exact credit will be. Just be prepared for a lower figure than you may expect.